Self generated protective atmosphere for liquid metals

ABSTRACT

An improved method of manufacturing a cast part by sand casting, permanent mold casting, investment casting, lost foam casting, die casting, or centrifugal casting, or a powder metal material by water, gas, plasma, ultrasonic, or rotating disk atomization is provided. The method includes adding at least one additive to a melted metal material before or during the casting or atomization process. The at least one additive forms a protective gas atmosphere surrounding the melted metal material which is at least three times greater than the volume of melt to be treated. The protective atmosphere prevents introduction or re-introduction of contaminants, such as sulfur (S) and oxygen (O 2 ), into the material. The cast parts or atomized particles produced include at least one of the following advantages: less internal pores, less internal oxides, median circularity of at least 0.60, median roundness of at least 0.60 and increased sphericity of microstructural phases and/or constituents.

CROSS REFERENCE TO RELATED APPLICATION

This U.S. utility patent application claims priority to U.S. provisionalpatent application No. 62/409,192, filed Oct. 17, 2016, the contents ofwhich is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates generally to metal materials, and moreparticularly to melted metal materials which are either atomized orsolidified to form powder metals or castings, and methods of forming thesame.

2. Related Art

Powder metal materials can be formed by various processes such as bywater atomization, gas atomization, plasma atomization, ultrasonicatomization or rotating disk. Powder metal materials are used in variousdifferent technologies such as pressed and sintered, metal injectionmolding, and additive manufacturing. Metal castings are also commonlyused in various technologies, including both automotive andnon-automotive parts, and produced by various processes such as by sandcasting, permanent mold casting, investment casting, lost foam casting,die casting, or centrifugal casting. Both atomization and castingprocesses begin with a melted metal material. Common atomizationprocesses include applying a fluid (water, gas, oil, ultrasonic, orplasma) to the melted metal material to form a plurality of particles.The casting process typically includes pouring the melted metal materialinto a mold having a desired shape, and allowing the liquid metal tosolidify before removing the metal part from the mold.

SUMMARY

One aspect of the invention provides a method of manufacturing a powdermetal material. This method includes adding at least one additive to amelted base metal material, the at least one additive forming aprotective gas atmosphere surrounding the melted base metal materialwhich has a volume of at least three times greater than the volume ofthe melted base metal material to be treated; and atomizing the meltedbase metal material after adding at least some of the at least oneadditive to produce a plurality of particles. Another aspect of theinvention provides a powder metal material formed from the melted metalmaterial with the self-generated protective atmosphere.

Another aspect of the invention provides a method of manufacturing acast part. The method includes adding at least one additive to a meltedbase metal material, the at least one additive forming a protective gasatmosphere surrounding the melted base metal material which has a volumeof at least three times greater than the volume of the melted base metalmaterial to be treated; and casting the melted metal material afteradding at least some of the at least one additive. Another aspect of theinvention provides a casting formed from the melted metal material withthe self-generated protective atmosphere.

Both methods include manufacturing a self-generated protectiveatmosphere in the melted base metal material. Adding the at least oneadditive to the melted base metal material can improve the quality ofthe melt. The at least one additive can create the protective atmospherewhich acts as a protective barrier against oxidation. The protectiveatmosphere also acts as a barrier to prevent impurities, such as sulfur(S) and/or oxygen (O₂), from entering or re-entering into the meltedmetal material. Thus, the at least one additive can limit oxidationduring the melting and pouring phases of the process and limit theamount of internal oxides. Additionally, physical structures of powderparticles and/or microstructural features in the solidified metalmaterial can be altered to improve or influence the material properties.For example, the at least one additive can also contribute to themicrostructural engineering of precipitate, such as size and morphology.

When the melted metal material is atomized, the at least one additivecan engineer the shape and morphology of the resulting powder particles.Also in the case of powder atomization, the at least one additiveimproves the roundness and sphericity of the resulting powder particles.The amount of internal porosities in powders and castings can also belowered.

The atomizing step can also include producing a plurality of particleshaving a spherical shape. The sphericity of the particles and that ofthe shape of microstructural phases or constituents in the atomizedparticles or castings in the as-atomized, as-cast or heat treated state,can be determined by two image analysis indicators, specificallycircularity and roundness, according to the following formulas:

Circularity (C)=4π×([Area]/[Perimeter]²)

Roundness (R)=4π([Area]/(π×[Major axis]²))=1/AR

wherein AR=[Major axis]/[Minor axis].

The image analysis indicators can be calculated using open sourcesoftware, such as ImageJ (http://imagej.nih.gov/ij/). A sphericity indexvalue of 1.0 indicates a perfect circle.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated,as the same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIGS. 1 to 3 present additives (cells marked with an “x”) that willcreate a protective gas atmosphere, those that will react with oxides,and those that will react with sulfur respectively for various chemicalsystems (Al, Cu, Mn, Ni, Co, Fe, Ti, and Cr);

FIG. 4 presents a curve of the calculated total volume of gas that isgenerated as a function of the amount of additive(s) for an examplecomposition;

FIG. 5 is a graph showing EDS spectra that were experimentally acquiredon a polished pure iron surface before and after it was exposed to theatmosphere on top of the tundish during an atomization process of thepowder that is described in FIG. 4;

FIG. 6 is a graph showing the calculated volume of gas generated bysodium (Na) and potassium (K) additives in aluminum at differenttemperatures (800 and 900 Celsius), wherein the dashed line shows theinferior limit of gas;

FIG. 7 is a graph showing the calculated volume of gas generated bydifferent additives in titanium at a temperature of 1800 Celsius,wherein the dashed line shows the inferior limit of gas;

FIG. 8 is a graph showing the calculated volume of gas generated bydifferent additives in cobalt at a temperature of 1600 Celsius, whereinthe dashed line shows the inferior limit of gas;

FIG. 9 is a graph showing the calculated volume of gas generated bydifferent additives in chromium at a temperature of 2000 Celsius,wherein the dashed line shows the inferior limit of gas;

FIG. 10 is a graph showing the calculated volume of gas generated bydifferent additives in copper at a temperature of 1200 Celsius, whereinthe dashed line shows the inferior limit of gas;

FIG. 11 is a graph showing the calculated volume of gas generated bydifferent additives in iron at a temperature of 1650 Celsius, whereinthe dashed line shows the inferior limit of gas;

FIG. 12 is a graph showing the calculated volume of gas generated bydifferent additives in manganese at a temperature of 1400 Celsius,wherein the dashed line shows the inferior limit of gas;

FIG. 13 is a graph showing the calculated volume of gas generated bydifferent additives in nickel at a temperature of 1600 Celsius, whereinthe dashed line shows the inferior limit of gas;

FIG. 14 is a graph showing the calculated total volume of gas that isobtained per 100 grams of melt of a complex cobalt alloy at atemperature of 1600 Celsius as a function of the amount of additive (Kand Li);

FIG. 15 is a backscattered electron micrograph of a water atomizedhypereutectic cast iron powder without added magnesium in which manyirregular primary graphite nodules precipitated on internal siliconoxides that were introduced in the melt during the pouring step of theatomization process;

FIG. 16 is a backscattered electron micrograph of another water atomizedhypereutectic cast iron powder with added magnesium in which onespherical primary graphite nodule precipitated on a heterogeneous oxidenuclei that contains Mg during the atomization process;

FIG. 17 is a backscattered electron micrograph of a water atomizedhypereutectic cast iron powder that contains about 4.0% C and 2.3% Siwithout added magnesium wherein graphite nodules which grew in the solidstate during a post heat treatment process are present;

FIG. 18 is a photomicrograph of another water atomized hypereutecticcast iron powder with added magnesium, according to an exampleembodiment, wherein more spherical graphite nodules compared to thosepresented in FIG. 17, which grew in the solid state during a post heattreatment process are present;

FIG. 19 illustrates the circularity frequency distribution of thegraphite nodules that were observed in the water atomized hypereutecticcast iron powders presented in FIGS. 17 and 18;

FIG. 20 illustrates the roundness frequency distribution of the graphitenodules that were observed in the water atomized hypereutectic cast ironpowders presented in FIGS. 17 and 18;

FIG. 21 is a table illustrating numerical data for the circularity ofthe graphite nodules that grew in the solid state for two hypereutecticcast iron powders that were observed in FIGS. 17 and 18;

FIG. 22 is a table illustrating numerical data for the roundness of thegraphite nodules that grew in the solid state for two hypereutectic castiron powders that were observed in FIGS. 17 and 18;

FIG. 23 is a backscattered electron micrograph of a water atomizedstainless steel powder without added magnesium screened at −80/+200 mesh(between 177 and 74 microns), wherein the red arrows point to internalporosities;

FIG. 24 is a backscattered electron micrograph of a another wateratomized stainless steel powder with added magnesium screened at−80/+200 mesh (between 177 and 74 microns), wherein one red arrow pointsto only one smaller internal porosity compared to those of FIG. 23;

FIG. 25 is an optical photomicrograph of a water atomized high carbonsteel alloyed with silicon powder that contains about 1.3% C and 1.1% Siwithout added magnesium screened at −200 mesh (74 microns and less)wherein the red arrows point to internal porosities;

FIG. 26 is an optical photomicrograph of a comparative water atomizedhigh carbon steel alloyed with silicon that contains about 1.4% C and1.1% Si with added magnesium screened at −200 mesh (74 microns and less)according to one example embodiment wherein the red arrows point tofewer internal porosities than the powder of FIG. 25; and

FIG. 27 includes a table listing compositions evaluated.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

One aspect of the invention includes an improved method of manufacturinga powder metal material by water or gas atomization or any otheratomization process that requires that the material to be atomized goesthrough the creation of a bath of liquid metal such as plasmaatomization, ultrasonic atomization or rotating disk atomization, byadding at least one additive to a melted metal material before and/orduring the atomization process. Another aspect of the invention includesan improved method of manufacturing a casting by processes such as sandcasting, permanent mold casting, investment casting, lost foam casting,die casting, or centrifugal casting from a melted metal material byadding at least one additive to the melted metal material. The at leastone additive forms a protective gas atmosphere surrounding the meltedmetal material which is at least three times greater than the volume ofmelt to be treated.

The protective atmosphere created by the at least one additive that isadded to the melted material acts as a barrier to prevent impurities,such as sulfur (S) and/or oxygen (O₂) or others, from entering orre-entering into the melted metal material by pushing them away from thesurface of the melted material as the protective gas is coming out ofthe melt. The additive(s) that forms the protective gas atmosphere canalso react with the dissolved sulfur in the melt and/or the oxides thatwere in suspension in the melt before the introduction of theadditive(s). Reaction of the additive(s) with the dissolved sulfur inthe melt will increase the sphericity of atomized particles formed fromthe melt and/or increase that of the microstructural phases andconstituents in the atomized particles or castings.

When water atomization is employed, adding the additive(s) to the meltedmetal material can increase the sphericity of the atomized particles toa level approaching the sphericity of particles formed by gasatomization, but with reduced costs compared to gas atomization. Addingthe additive(s) to the melted metal material can also produce cleanerparticles by limiting the formation and the entrainment of new oxidesfrom the surface of the melt and by reacting with those already presentin the melt before the introduction of the additive(s). These oxides canform as bifilms where films of oxides are folded on themselves leaving aweak interface in between the oxide films. The additive(s) can alsolower the amount and size of internal porosity, a problem encountered inatomized powders. The additive(s) can also increase the sphericity ofmicrostructural constituents and/or phases formed in the atomizedparticles and/or during a subsequent heat treatment process. Forexample, if the atomized particles are formed from a cast iron material,at least 50% of the graphite precipitates formed during the post heattreatment process will have a circularity of at least 0.6 and aroundness of at least 0.6.

When casting is employed, adding the additive(s) to the melted metalmaterial can increase the sphericity of microstructural constituentsand/or phases formed in the castings and/or during a subsequent heattreatment process. Adding the additive(s) to the melted metal materialcan also produce cleaner castings by limiting the formation and theentrainment of new oxides from the surface of the melt and by reactingwith those already present in the melt before the introduction of theadditive(s). These oxides can form as bifilms where films of oxides arefolded on themselves leaving a weak interface in between the oxidefilms. The additive(s) can also lower the amount and size of internalporosity, a problem encountered in many castings.

According to one example embodiment, the method begins by melting a basemetal material. Many different metal compositions can be used as thebase metal material. However, in order to produce enough gas that willact as a protective atmosphere and thus obtain either the desiredspherical-shape of the powders and/or more spherical microstructuralconstituents and/or cleaner particles and /or having less internalpores, the additive(s) must have a low solubility in the metal material.The base material and the additive(s) should be selected such that whenthe additive(s) are introduced, the volume of protective gas atmospheregenerated is at least three times the volume of melted metal material tobe treated. For example, if 0.22 weight percent (wt. %) magnesium isadded to an iron-rich melt, the generated volume of gas will be about 20times the inferior limit of gas required to provide a protectiveatmosphere which is defined as three times the initial volume of melt tobe treated.

The base metal material typically includes at least one of aluminum(Al), copper (Cu), manganese (Mn), nickel (Ni), cobalt (Co), iron (Fe),titanium (Ti), and chromium (Cr). The base metal material can comprisepure Al, Cu, Mn, Ni, Co, Fe, Ti, or Cr. Aluminum-rich, copper-rich,manganese-rich, nickel-rich, cobalt-rich, iron-rich, titanium-rich andchromium-rich alloys, or an alloy including at least 50 wt. % of Al, Cu,Mn, Ni, Co, Fe, Ti, and/or Cr are also well suited for use as thestarting base metal material. Mixtures of these base metal materials indifferent proportions are also well suited for use as the startingmaterial such as, but not limited to, Al—Cu, Fe—Ni, Fe—Co, Fe—Ni—Co,Ni—Cr, Ti—Cu, and Co—Cr alloys. The alloys can also include at least oneof the following as alloying elements, as long as they will stay insolution in the melt of the alloy of interest: silver (Ag), boron (B),barium (Ba), beryllium (Be), carbon (C), calcium (Ca), cerium (Ce),gallium (Ga), germanium (Ge) potassium (K), lanthanum (La), lithium(Li), magnesium (Mg), molybdenum (Mo), nitrogen (N), sodium (Na),niobium (Nb), phosphorus (P), sulfur (S), scandium (Sc), silicon (Si),tin (Sn), strontium (Sr), tantalum (Ta), vanadium (V), tungsten (W),yttrium (Y), zinc (Zn), and zirconium (Zr).

There is a distinction to be made between the elements described as“alloying elements” and those described as “additives.” Alloyingelements will stay in solution in the base metal material and/or formdifferent phases/constituents in the final parts/powders.

Alloying elements will impact the microstructure and the properties ofthe parts. For instance, C in Fe will form cementite, which increasesstrength. Additives are defined as elements added to the melt to eithercreate a protective gas atmosphere, react with S and/or with oxides.FIGS. 1 to 3 include a complete list of additives in different basemetal materials. One particular element can be an alloying element inone base material but be an additive in a different base material. Forinstance, Mg is an alloying element in Al-rich alloys but is an additivein Fe-rich alloys. According to one example embodiment, to create agaseous protective atmosphere in an Al-Mg alloy, K and/or Na should beused as an additive and the melt temperature should be selectedaccording to the selected additive(s). For example, since Mg is used asan alloying element in aluminum alloys (the Al-5000 series) it will notgenerate a protective gas atmosphere.

However, the starting metal material is not limited to the abovementioned compositions. Other metal compositions can be used, as long asthe additive has a low solubility in the selected material and generatesa sufficient amount of protective gas atmosphere. Some additives thatare used to create the gaseous protective atmosphere will naturallyreact with the dissolved sulfur in the melt to create more stablecompounds and thus increase the surface tension. This is the case for Mgin Fe-rich systems in which solid MgS will precipitate. However, someadditives will create a protective atmosphere but will not react withthe dissolved sulfur, as is the case with Na in Fe-rich systems. Inthese situations, a combination of different additives must be used toincrease surface tension and create a protective atmosphere.

As mentioned above, various different additives could be added to themelted metal material to achieve the increased protective atmosphere andthe other advantages mentioned above. The additive(s) selected dependson the composition of the base metal material. For example the at leastone additive can include at least one of K, Na, Zn, Mg, Li, Ca, Sr, andBa. The protective gas atmosphere generated by the additive(s) preventsimpurities from entering or re-entering into the melted metal material.

The additives listed above generate different amounts of protective gasatmosphere, depending on the chemical system in which they are used.Some additives are more suited for some systems than others. Forexample, in aluminum alloys, K and Na are oftentimes preferred. Incopper alloys, K and Na are oftentimes preferred. In manganese alloys,K, Na, Zn, Mg, and Li are oftentimes preferred. In nickel alloys, K andNa are oftentimes preferred. In cobalt alloys, K, Na, Li, and Ca areoftentimes preferred. In iron alloys, K, Na, Zn, Mg, Li, Sr, and Ca areoftentimes preferred. In titanium alloys, Zn, Mg, Li, Ca, and Ba areoftentimes preferred. In chromium alloys, K, Na, Zn, Mg, Li, Sr, Ca, andBa are oftentimes preferred. Examples are provided in FIG. 1, whereinthe preferred additives are marked.

According to one specific example embodiment, the metal base material isiron-rich and includes Mg which generates the protective gas and alsoreacts with the sulfur impurity. Alternatively, the base metal materialis pure iron and the additive is Mg. According to another specificexample, the metal base material is iron-rich and the additives includea mixture of K and Ba. The potassium (K) will generate the protectivegas atmosphere, and the barium (Ba) will react with the sulfur.

The protective atmosphere limits the amount of oxides in the atomizedparticles and castings and will also limit the size and amount ofinternal porosities in the atomized particles and castings. Someadditives that are used to create the gaseous protective atmosphere willnaturally react with oxides that are in suspension in the melt to createmore stable compounds and will also change their morphology during thechemical reaction process, for example a Mg additive in Fe-rich systemsthat contain Si as an alloying element. In these materials, oxides ofSiO₂ that could be in the form of bifilms (overlapping films of oxidesthat are poorly bounded) are in suspension in the melt. One of thereason explaining that a smaller amount of porosities is observed isthat Mg helps to bound the interfaces between the overlapping films, aresult of a chemical reaction between Mg and the oxides, creating astronger interface that cannot be subsequently separated to form pores.The self-generated Mg gaseous atmosphere will limit further oxidation ofthe surface of the melt, which will limit the amount of internal oxidesin the particles. However, some additives will create a protectiveatmosphere but will not react with the oxides in suspension in the melt,as is the case of Zn in Ti-rich systems. In these situations, acombination of different additives must be used to limit the amount andsize of internal porosities. For example, at least one additive could beadded to generate the protective gas atmosphere that will preventimpurities from entering or re-entering into the melted metal material,and at least one additive could be added to react with the oxidesalready in the melt but would not necessarily create a protective gasatmosphere. An example of such a combination of additives in a Ti-richalloy to create more spherical particle and/or phases and constituentshaving less internal porosities could be a mixture of Zn to create aprotective atmosphere and Sr to react with S and with TiO₂ but withoutparticipating in the generation of the protective atmosphere.

In other words, some additives are more effective in some systems thanin others, depending on the type of oxides that are formed. As indicatedabove, if less internal porosities with smaller sizes are desired, theadditive(s) must react with the oxides in suspension in the melt. Theseoxides are also considered impurities in the melted base metal material,for example, Al₂O₃ in an aluminum-based material, or Fe₂O₃ in aniron-based material. When the melted base metal material is an aluminumalloy or aluminum-based, the preferred additives to react with theoxides include K, Na, Mg, Li, and Ca. When the melted base metalmaterial is an iron alloy or iron-based, the preferred additives toreact with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. Whenthe melted base metal material is a titanium alloy or titanium-based,the preferred additives to react with the oxides include Sr, Ca, and Ba.When the melted base metal material is a chromium alloy orchromium-based, the preferred additives to react with the oxides includeK, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the melted base metal materialis a cobalt alloy or cobalt-based, the preferred additives to react withthe oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the meltedbase metal material is a copper alloy or copper-based, the preferredadditives to react with the oxides include K, Na, Zn, Mg, Li, Sr, Ca,and Ba. When the melted base metal material is a manganese alloy ormanganese-based, the preferred additives to react with the oxidesinclude K, Na, Zn, Mg, Li, Sr, Ca, and Ba. When the melted base metalmaterial is a nickel alloy or nickel-based, the preferred additives toreact with the oxides include K, Na, Zn, Mg, Li, Sr, Ca, and Ba.Examples are provided in FIG. 2.

When the melted base material is iron-based and includes sulfur as animpurity, Zn, Mg, Li, Sr, Ca, and Ba are preferred to react with thesulfur. An example of such a combination of additives in an iron-basedmaterial or Fe-rich alloy to create more spherical particle and/orphases and constituents could be a mixture of Na and Ba. Na will createa protective atmosphere and Ba to will react with S. When the meltedbase metal material is a titanium alloy or titanium-based and includessulfur as an impurity, K, Na, Zn, Mg, Li, Sr, Ca, and Ba are preferredto react with the sulfur. When the melted base metal material is acobalt alloy or cobalt-based and includes sulfur as an impurity, Na, Mg,Li, Sr, Ca, and Ba are preferred to react with the sulfur. When themelted base metal material is a chromium alloy or chromium based andincludes sulfur as an impurity, K, Na, Zn, Mg, Sr, Ca, and Ba arepreferred to react with the sulfur. When the melted base metal materialis an aluminum alloy or aluminum-based and includes sulfur as animpurity, K, Na, Mg, Li, Sr, Ca, and Ba are preferred to react with thesulfur. When the melted base metal material is a nickel alloy ornickel-based and includes sulfur as an impurity, Mg, Li, Sr, Ca, and Baare preferred to react with the sulfur. When the melted base metalmaterial is a copper alloy or copper-based and includes sulfur as animpurity, K, Na, Mg, Li, Sr, Ca, and Ba are preferred to react with thesulfur. When the melted base metal material is a manganese alloy ormanganese-based and includes sulfur as an impurity, K, Na, Mg, Li, Sr,Ca, and Ba are preferred to react with the sulfur. Examples are providedin FIG. 3.

In addition, certain additives will successfully generate the protectivegas atmosphere, and also react with the sulfur and oxides present asimpurities in the melted base metal material. For example, when themelted base metal material is an iron-alloy or iron-based, additivesthat will generate the protective gas atmosphere and react with thesulfur and oxide impurities include Zn, Mg, Li, Sr, and Ca. When themelted base metal material is a titanium alloy or titanium-based,additives that will generate the protective gas atmosphere and reactwith the sulfur and oxide impurities include Ca and Ba. When the meltedbase metal material is a chromium alloy or chromium-based, additivesthat will generate the protective gas atmosphere and react with thesulfur and oxide impurities include K, Na, Zn, Mg, Sr, Ca, and Ba. Whenthe melted base metal material is a cobalt alloy or cobalt-based,additives that will generate the protective gas atmosphere and reactwith the sulfur and oxide impurities include Na, Li, and Ca. When themelted base metal material is an aluminum alloy or aluminum-based,additives that will generate the protective gas atmosphere and reactwith the sulfur and oxide impurities include K and Na. When the meltedbase metal material is a copper alloy or copper-based, additives thatwill generate the protective gas atmosphere and react with the sulfurand oxide impurities include K and Na. When the melted base metalmaterial is a manganese alloy or manganese-based, additives that willgenerate the protective gas atmosphere and react with the sulfur andoxide impurities include K, Na, Mg, and Li.

As stated above, the melted metal material can be atomized, for exampleby water or gas atomization, to form powder metal. Alternatively, themelted metal material can be formed into a casting.

As alluded to above, the starting base metal material selectedoftentimes includes iron in an amount of at least 50.0 wt. %, based onthe total weight of the metal material before adding the additive(s).For example, cast irons, highly alloyed cast irons, stainless steels,unalloyed and alloyed steels, tool steels, Maraging steels, or Hadfieldsteels could be used. According to one example embodiment, the metalmaterial is a steel powder including 1.3 wt. % carbon and 1.1 wt. %silicon. According to another example embodiment, the metal material isa cast iron powder including 4.0 wt. % carbon and 2.3 wt. % silicon.According to another example embodiment, the metal material is astainless steel powder including 1.2% Mn, 0.30% Si, 0.44% Cu, 0.23% Mo,17.3% Cr, 9.5% Ni, and other trace elements. As stated above, aluminumalloys (for instance the alloys designated as 2024, 3003, 3004, 6061,7075, 7475, 5080 and 5082), copper alloys (such as aluminum bronzes,silicon bronzes, and brass), manganese alloys, nickel alloys (forinstance the alloy designated as 625), cobalt alloys (such as tribaloyand Haynes188), cobalt-chromium alloys (such as CoCrMo alloys andstellite), titanium alloys (for instance the alloys designated asTi-6Al-4V or as Ti-6Al), chromium alloys (such as the Kh65NVFT alloy)and any hybrid alloys made from these chemical systems can also be usedas the starting powder metal material (for instance, alloys designatedas Invar, Monel, Chromel, Alnico, and Nitinol60). These examples are notexhaustive and other metal compositions can be used, as long as the atleast one additive (potassium (K), sodium (Na), zinc (Zn), magnesium(Mg), lithium (Li), strontium (Sr), calcium (Ca), and barium (Ba)) haslow solubility in the selected material, such that a protective gasatmosphere is formed on top of the melted material to form a totalamount of at least three times the initial volume of melt to be treated.FIGS. 4-14 represent the results of calculations and experimentsconducted which show the increased volume of protective gas atmospheregenerated when the additive(s) are added to the melted metal materialaccording to example embodiments of the invention. FIG. 4 presents acurve of the total volume of gas that is obtained as a function of theamount of additive(s) for an example composition. The additive (here,the additive was a mixture of 90 wt.% Mg and 10 wt.% Na). The alloy is acast iron powder material (Fe-rich) that contains 4.0% C, 1.5% Si, 0.02%S and 2.0% Cu. This curve was calculated using the chemical compositionof one powder that was water atomized, the amount of additive used inthis experiment was 0.11 wt. %, which resulted in about 0.40 liter ofprotective gas (Mg and Na) for each 100 grams of melt. The dashed linerepresents the inferior limit of gas that should be obtained to providea protective atmosphere which is a volume that is three times theinitial volume of melt to be treated. In this specific example, thecalculated amount of gas is about five times the inferior limit.

FIG. 5 presents Energy-dispersive X-ray spectroscopy (EDS) spectra thatwere acquired on a polished pure iron surface before and after it wasexposed to the gaseous atmosphere on top of the tundish during theatomization process of the powder that is described in FIG. 4. Thisconfirms that the additives (in this case Mg and Na) formed a gaseousprotective atmosphere that was generated on top of the melt and thatthese elements deposited on the exposed polished iron surface;

FIG. 6 presents examples of different amounts of gas that can begenerated in aluminum alloys for different additives at differenttemperatures. The base system for calculations is Al+0.02% S+0.02%Al2O3. The dashed line represents the inferior limit of the amount ofgas that should be obtained to provide a protective atmosphere which isdefined as three times the initial volume of melt to be treated. Inthese examples, the minimum amount of additive to be added variesaccording to the nature of the additive and the temperature of the melt.For instance, Na cannot generate enough gas if the melt is at atemperature of about 800 Celsius, regardless of the amount that isadded. However, if the temperature of the melt is increased to about 900Celsius, the minimum amount of Na is about 0.32 wt. % to generate atleast three times the initial volume of melt to be treated. For K, theminimum amount is 0.36 wt. % if the melt is at 800 Celsius, and 0.26 wt.% if the melt is at about 900 Celsius. If a mixture of half Na and halfK is used in an aluminum melt at 900 Celsius, the minimum amount of Na+Kwill be about 0.29 wt. % (0.16 wt. % Na and 0.13 wt. % K). FIG. 7presents examples of the minimum amount of different additives to beadded to a titanium melt at 1800 Celsius. For instance, an addition of0.11 wt. % Ca will provide about the same minimum amount of gasprotection as an addition of 0.48 wt. % Zn. Similarly, FIGS. 8 to 13present other examples of the minimum amount of different additives indifferent systems (Co, Cr, Cu, Fe, Mn, and Ni). FIG. 14 presents thecalculated minimum amount of additive (K+Li) in a complex cobalt alloy.

After adding the at least one additive to the melted base metalmaterial, the melt can be either atomized or cast. Water atomization isoftentimes preferred to gas atomization because it is three to ninetimes less expensive and is even less expensive than the otheratomization processes. However, for some alloys that are readilyoxidized, gas atomization is preferred. An additive treatment before gasatomization could allow improved conditions for atomization such aslarger gas pressures and still achieve round particles and could alsolimit the amount of internal oxides and porosities. In addition, theadded additive(s) can increase the sphericity of the water atomizedparticles, such that the sphericity approaches the sphericity of gasatomized particles.

As discussed above, the additive(s) is added in an amount such that thetotal volume of gas after the introduction of the additive(s) is atleast three times the initial volume of the melt to be treated. In oneexample embodiment, the additive, in this case, Mg, is added in a singleoperation as lumps of pure Mg in an amount ranging from 0.05 to 1.0 wt.%, for example 0.18 wt. %, based on the total weight of the melted basemetal material (an iron-rich alloy) and the added magnesium. Thus, theresulting atomized powder metal material or casting includes a very lowamount of residual magnesium and a total sulfur content similar to thematerial without the additive but for which S is now chemically boundedwith the additive (as solid precipitates of MgS) and not dissolved inthe melt, which leads to a larger surface tension and thus morespherical particles, and/or more spherical microstructural phases andconstituents, and/or a lower amount of internal porosities.Thermodynamical calculations showed that the free sulfur content in theMg-treated iron-rich material was more than 10 times lower than that ofthe non-treated material, even if the total sulfur content for bothmaterial was similar.

The additive(s) can be added in a single continuous step, for example upto 1.0 wt. % in a single continuous step, or multiple steps spaced fromone another by a period of time, for example three or four steps eachincluding up to 0.2 wt. % of the additive(s). The additive(s) can alsoor alternatively be added in the furnace or in a ladle and they can bein the form of pure metal, or as an alloy or compound including theadditive(s). Different techniques that are already available can be usedto introduce the additive(s) to the melted metal materials such as, butnot limited to, lumps/chunks of the material that contain theadditive(s) can be directly deposited on top of the melt or at thebottom of the furnace/crucible, or in the mold, or introduced in themelt by the usage of the cored wire technique or the usage of theplunger process. For instance, the cored wire technique uses a steelsheath filled with the Mg-rich alloy and is introduced in the melt at arate dependent on the process parameters. The plunger technique uses acontainer in which the Mg containing master alloy is located, thiscontainer is plunged into the liquid cast iron. Therefore, magnesiummakes contact with the liquid cast iron deeper into the melt, away fromthe surface.

As stated above, by adding the additive(s) to the melted metal material(in the case of Mg in Fe-rich alloys), the number of water atomizedparticles that have a circularity and a roundness value of 0.6 andlarger increased by at least 8%, compared to the same water atomizedmaterial without the additive(s). The additive(s), for examplemagnesium, also results in fewer internal oxides, and could close theinterface of residual oxide bifilms present in the melted metalmaterial. This, in turn, produces cleaner atomized particles and cleanercastings having less and smaller internal porosities.

After the atomization or casting step, the method can include a postheat treatment process. The heat treating step can include annealing oranother heating process typically applied to powder metal materials. Theheat treatment can be conducted in an inert or reducing atmosphere, suchas but not limited to an atmosphere including nitrogen, argon, and/orhydrogen or vacuum. For example, annealing in a reducing atmosphereafter water atomization can reduce surface oxides. The heat treatmentstep can also be used to form new microstructural phases and/orconstituents in the atomized particles or castings, for example graphiteprecipitates or nodules, carbides, or nitrides. Other microstructuralphases and/or constituents could be present, depending on thecomposition of the metal material. In one example embodiment, the metalmaterial is a hypereutectic cast iron alloy, and the cementite presentin the cast iron alloy transforms into ferrite and spheroidal graphitenodules during the heat treatment step, see FIGS. 17 and 18. Sphericalcarbides should also be formed during the heat treatment of highlyalloyed steel. An external protective atmosphere or vacuum system canalso be used together with the self-generated protective atmospheredescribed herein such as, but not limited to: the projection of a flowof nitrogen (N2), or the projection of an argon (Ar) stream on top ofthe melt. The melt could also be enclosed in a chamber with a protectiveinert atmosphere or a vacuum system. These systems can increase theeffectiveness of the process.

The additive(s) can also increase the sphericity of the microstructuralconstituents and/or phases formed in the atomized particles or castingsduring post heat treatment. However, rounder phases and/or constituentscould be present in the powder metal material directly after atomizationor in the as-cast materials and not only after heat treatments. Themicrostructural phases can include graphite precipitates, carbides,and/or nitrides. Other microstructural phases and/or constituents couldbe present, depending on the composition of the metal material.Typically, the microstructural constituents and/or phases have a medianof the circularity and a median of the roundness of at least 0.6. Also,there is at least 10% more, and preferably at least 15% moreconstituents and/or phases formed in the magnesium-treated iron-basedmaterial that have a circularity and a roundness value larger than 0.6compared to those of the same alloy but without the additive treatment.

According to one example embodiment, the powder metal material includesiron, such as cast iron, in an amount of at least 50 wt. %, and theatomized particles include graphite precipitates, wherein at least 50%of the graphite precipitates have a circularity and a roundness value of0.6 and greater. In another embodiment, wherein the metal material isiron-based and was treated with Mg, the annealing step includesproducing graphite precipitates or nodules, and the graphiteprecipitates or nodules have a median of the circularity and a median ofthe roundness of at least 0.6. In one example embodiment, the metalmaterial is a hypereutectic cast iron alloy treated with Mg, andspheroidal graphite nodules are formed during the heat treatmentprocess.

As stated above, the self-generated protective atmosphere created afterthe introduction of the additive(s) will inhibit the oxidation of thesurface of the melt and will limit the amount of internal oxides inpowders after atomization and in castings after solidification. FIG. 15shows primary graphite nodules in a hypereutectic cast iron powder thatprecipitated on silicon oxides in suspension in the melt that wereformed during pouring from the crucible to the tundish; this alloy wasnot treated with any additives. In Fe-rich systems that contain a highcarbon content, carbon provides a protection against oxidation of themelt in the crucible (because of the high temperature), which preventsthe formation of oxides in the crucible. Numerous graphite nodules thatgrew on these different oxides can be observed in the powder without anadditive. By comparison, FIG. 16 presents one of the relatively fewprimary graphite nodules that can be observed in the hypereutectic castiron powder that was treated with an additive (Mg). Since the protectiveatmosphere made of Mg gas limited the oxidation of the melt directlyfrom the crucible and throughout pouring, the amount of oxides that werepresent in the melt before the introduction of the additive wassignificantly less than in the melt without the additive. Thus, very fewsubstrates were available for graphite precipitation duringsolidification and fewer graphite nodules are present.

As stated above, the melted metal material can be atomized to form apowder metal material or cast to form a solidified part. The powdermetal material is typically formed by water or gas atomization, howeveranother atomization process can be used. Powders and castings obtainedwith the disclosed method can be used in various different automotive ornon-automotive applications. For example, the atomized particles can beused in typical press and sinter processes. The atomized particles canalso be used for metal injection molding, thermal spraying, and additivemanufacturing applications such as three-dimensional printing, electronbeam melting, binder jetting and selective laser sintering.

When the melted metal material is cast, the method includes melting thebase metal material, and then adding the at least one additive to thebase metal material. The method then includes pouring the melted metalmaterial into a mold having a desired shape, and allowing the liquidmetal to solidify before removing the solidified metal part from themold.

Experiment

FIGS. 17 and 18 are photomicrographs illustrating the improvedsphericity of the microstructural phases and/or constituents,specifically graphite nodules, achieved by adding an additive (in thiscase magnesium) before or during the water atomization process and afterheat treatment. Each material is a cast iron powder including about 4.0wt. % carbon and 2.3 wt. % silicon. However, the material of FIG. 17 wasatomized without the added magnesium, while the material of FIG. 18 wasatomized with the added magnesium. The median of the roundness of thegraphite nodule shown in FIG. 17, without the added magnesium, wascalculated to be 0.56. The median of the roundness of the graphitenodule with magnesium shown in FIG. 18, was calculated to be 0.73. Otherresults that show the improved sphericity of the nodules by the additivetreatment are presented in FIGS. 19 to 22.

FIGS. 23 and 24 illustrate the lower internal porosities contentaccording to an example embodiment of the invention. In this example 304stainless steels were water atomized. The powder presented in FIG. 24was treated with Mg and showed a lower amount of internal porosities.

FIGS. 25 and 26 illustrate the lower internal porosities contentaccording to an example embodiment of the invention. In this examplehigh carbon steels alloyed with silicon were water atomized. The powderpresented in FIG. 26 was treated with Mg and showed a lower amount ofinternal porosities.

FIG. 27 presents the chemical composition of the example embodiments ofthe invention.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings and may be practicedotherwise than as specifically described while within the scope of thefollowing claims. In particular, all features of all claims and of allembodiments can be combined with each other, as long as they do notcontradict each other.

What is claimed is:
 1. A method of manufacturing a powder metalmaterial, comprising the steps of: adding at least one additive to amelted base metal material, the at least one additive forming aprotective gas atmosphere surrounding the melted base metal materialwhich has a volume of at least three times greater than the volume ofthe melted base metal material to be treated; and atomizing the meltedbase metal material after adding at least some of the at least oneadditive to produce a plurality of particles.
 2. The method of claim 1,wherein the melted base metal material is iron-based, and the at leastone additive includes magnesium.
 3. The method of claim 1, wherein theatomizing step includes water atomizing, gas atomizing, plasmaatomizing, ultrasonic atomization or rotating disk atomizing.
 4. Themethod of claim 1, wherein the melted base metal material includes atleast one of aluminum (Al), copper (Cu), manganese (Mn), nickel (Ni),cobalt (Co), iron (Fe), titanium (Ti), and chromium (Cr); and the meltedbase metal material optionally contains at least one alloying elementselected from the group consisting of silver (Ag), boron (B), barium(Ba), beryllium (Be), carbon (C), calcium (Ca), cerium (Ce), gallium(Ga), germanium (Ge) potassium (K), lanthanum (La), lithium (Li),magnesium (Mg), molybdenum (Mo), nitrogen (N), sodium (Na), niobium(Nb), phosphorus (P), sulfur (S), scandium (Sc), silicon (Si), tin (Sn),strontium (Sr), tantalum (Ta), vanadium (V), tungsten (W), yttrium (Y),zinc (Zn), and zirconium (Zr).
 5. The method of claim 4, wherein the atleast one additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca,and Ba.
 6. The method of claim 4, wherein the melted base metal materialis iron-based, and the at least one additive forming the protective gasatmosphere includes at least one of K, Na, Zn, Mg, Li, Sr, and Ca. 7.The method of claim 4, wherein the melted base metal material isiron-based and includes sulfur present as an impurity; and the at leastone additive includes at least one of Zn, Mg, Li, Sr, Ca, and Ba.
 8. Themethod of claim 4, wherein the melted base metal material is iron-basedand includes at least one oxide present as an impurity; and the at leastone additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca, and Ba.9. The method of claim 4, wherein the melted base metal material isiron-based and includes sulfur and at least one oxide present asimpurities; and the at least one additive forming the protective gasatmosphere includes at least one of Zn, Mg, Li, Sr, and Ca.
 10. Themethod of claim 4, wherein the melted base metal material is an aluminumalloy and includes sulfur and/or at least one oxide present asimpurities; the at least one additive forming the protective gasatmosphere includes at least one of K and Na; and the at least oneadditive includes at least one of K, Na, Mg, Li, Sr, Ca, and Ba to reactwith the sulfur, and/or the at least one additive includes at least oneof K, Na, Mg, Li, Ca to react with the at least one oxide.
 11. Themethod of claim 4, wherein the melted base metal material istitanium-based and includes sulfur and/or at least one oxide present asimpurities; the at least one additive forming the protective gasatmosphere includes at least one of Zn, Mg, Li, Ca and Ba; and the atleast one additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca,and Ba to react with the sulfur, and/or the at least one additiveincludes at least one of Sr, Ca, and Ba to react with the at least oneoxide.
 12. The method of claim 4, wherein the melted base metal materialis a cobalt alloy and includes sulfur and/or at least one oxide presentas impurities; the at least one additive forming the protective gasatmosphere includes at least one of K, Na, Li and Ca; and the at leastone additive includes at least one of Na, Mg, Li, Sr, Ca, and Ba toreact with the sulfur, and/or the at least one additive includes atleast one of K, Na, Zn, Mg, Li, Sr, Ca, Ba to react with the at leastone oxide.
 13. The method of claim 4, wherein the melted base metalmaterial is a chromium alloy and includes sulfur and/or at least oneoxide present as impurities; the at least one additive forming theprotective gas atmosphere includes at least one of K, Na, Zn, Mg, Li,Sr, Ca and Ba; and the at least one additive includes at least one of K,Na, Zn, Mg, Sr, Ca, and Ba to react with the sulfur, and/or the at leastone additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca, and Bato react with the at least one oxide.
 14. The method of claim 4, whereinthe melted base metal material is iron-based; and the at least oneadditive includes Mg.
 15. A method of manufacturing a cast part,comprising the steps of: adding at least one additive to a melted basemetal material, the at least one additive forming a protective gasatmosphere surrounding the melted base metal material which has a volumeof at least three times greater than the volume of the melted base metalmaterial to be treated; and casting the melted metal material afteradding at least some of the at least one additive.
 16. The method ofclaim 15, wherein the melted base metal material is iron-based, and theat least one additive includes magnesium.
 17. The method of claim 15,wherein the casting step includes sand casting, permanent mold casting,investment casting, lost foam casting, die casting, or centrifugalcasting.
 18. The method of claim 15, wherein the melted base metalmaterial includes at least one of aluminum (Al), copper (Cu), manganese(Mn), nickel (Ni), cobalt (Co), iron (Fe), titanium (Ti), and chromium(Cr); and the melted base metal material optionally contains at leastone alloying element selected from the group consisting of silver (Ag),boron (B), barium (Ba), beryllium (Be), carbon (C), calcium (Ca), cerium(Ce), gallium (Ga), germanium (Ge) potassium (K), lanthanum (La),lithium (Li), magnesium (Mg), molybdenum (Mo), nitrogen (N), sodium(Na), niobium (Nb), phosphorus (P), sulfur (S), scandium (Sc), silicon(Si), tin (Sn), strontium (Sr), tantalum (Ta), vanadium (V), tungsten(W), yttrium (Y), zinc (Zn), and zirconium (Zr).
 19. The method of claim18, wherein the at least one additive includes at least one of K, Na,Zn, Mg, Li, Sr, Ca, and Ba.
 20. The method of claim 18, wherein themelted base metal material is iron-based, and the at least one additiveforming the protective gas atmosphere includes at least one of K, Na,Zn, Mg, Li, Sr, and Ca.
 21. The method of claim 18, wherein the meltedbase metal material is iron-based and includes sulfur present as animpurity; and the at least one additive includes at least one of Zn, Mg,Li, Sr, Ca, and Ba.
 22. The method of claim 18, wherein the melted basemetal material is iron-based and includes at least one oxide present asan impurity; and the at least one additive includes at least one of K,Na, Zn, Mg, Li, Sr, Ca, and Ba.
 23. The method of claim 18, wherein themelted base metal material is iron-based and includes sulfur and atleast one oxide present as impurities; and the at least one additiveforming the protective gas atmosphere includes at least one of Zn, Mg,Li, Sr, and Ca.
 24. The method of claim 18, wherein the melted basemetal material is an aluminum alloy and includes sulfur and/or at leastone oxide present as impurities; the at least one additive forming theprotective gas atmosphere includes at least one of K and Na; and the atleast one additive includes at least one of K, Na, Mg, Li, Sr, Ca, andBa to react with the sulfur, and/or the at least one additive includesat least one of K, Na, Mg, Li, Ca to react with the at least one oxide.25. The method of claim 18, wherein the melted base metal material istitanium-based and includes sulfur and/or at least one oxide present asimpurities; the at least one additive forming the protective gasatmosphere includes at least one of Zn, Mg, Li, Ca and Ba; and the atleast one additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca,and Ba to react with the sulfur, and/or the at least one additiveincludes at least one of Sr, Ca, and Ba to react with the at least oneoxide.
 26. The method of claim 18, wherein the melted base metalmaterial is a cobalt alloy and includes sulfur and/or at least one oxidepresent as impurities; the at least one additive forming the protectivegas atmosphere includes at least one of K, Na, Li and Ca; and the atleast one additive includes at least one of Na, Mg, Li, Sr, Ca, and Bato react with the sulfur, and/or the at least one additive includes atleast one of K, Na, Zn, Mg, Li, Sr, Ca, Ba to react with the at leastone oxide.
 27. The method of claim 18, wherein the melted base metalmaterial is a chromium alloy and includes sulfur and/or at least oneoxide present as impurities; the at least one additive forming theprotective gas atmosphere includes at least one of K, Na, Zn, Mg, Li,Sr, Ca and Ba; and the at least one additive includes at least one of K,Na, Zn, Mg, Sr, Ca, and Ba to react with the sulfur, and/or the at leastone additive includes at least one of K, Na, Zn, Mg, Li, Sr, Ca, and Bato react with the at least one oxide.
 28. The method of claim 18,wherein the melted base metal material is iron-based; and the at leastone additive includes Mg.